1. Field of the Invention
The present invention relates to a cathode ray tube whose velocity modulating effect is enhanced and more specifically to a cathode ray tube which enables to display high quality images having remarkable contrast by preventing the decrease of velocity modulating effect caused by eddy current caused by velocity modulating magnetic field generated in electrodes composing an electron gun.
2. Related Art
Various contrivances for displaying high definition and high contrast images have been made to improve the imaging quality of a cathode ray tube for displaying TV images and of a cathode ray tube of an information terminal such as a personal computer.
For instance, there has been known an aperture compensating method of stressing white components by a signal obtained by differentiating an image signal to clearly display outlines. However, this method has had drawbacks that there is a case when it deteriorates image quality in contrary by generating unnecessary white peaks and deteriorating the contrast and that only the right side (downstream side of the horizontal scan direction) of the contrast boundary of an image can be always corrected.
There has been also a velocity modulation of changing electron beam scanning velocity corresponding to the brightness level of an image. This method is to control the scan of an electron beam. The scan of electron beam stop momently after quickening the scanning velocity momently when the electron beam scans horizontally from the black level to the white level by the differential output of the image signal. The scan of electoron beam quicken momently after stopping the scan momently when the electron beam scans horizontally from the white level to the black level.
The density of electron beam is low and the image is dark at the spot where the scan rate is fast. The density of electron beam is high and the image is bright at the spot where the scan stops. Thereby, a high contrast and good quality image is displayed by increasing the black level areas, by narrowing the white level area and by increasing the brightness by increasing the current density.
While there are electrostatic and electromagnetic type velocity modulations, a cathode ray tube using the electromagnetic type velocity modulation which has been currently widely adopted will be explained below.
FIG. 13 is a diagrammatic sectional view for explaining a structural example of the main part of the cathode ray tube which adopts the conventional electromagnetic type velocity modulation and which comprises a cathode K, a first electrode 1 (control electrode), a second electrode 2 (first accelerating electrode), a third electrode 3 (second accelerating electrode), a fourth electrode 4 (focusing electrode) and a fifth electrode 5 (anode electrode).
The cathode ray tube has a panel portion (not shown) having a phosphor screen and a vacuum envelope comprising a funnel portion 22 and a neck portion 23. An electron gun is housed within the neck portion 23 and a deflecting yoke 30 is externally mounted around the transition area between the neck portion 23 and the funnel portion 22.
The cathode ray tube also has a convergence regulating and color purity regulating correcting magnetic device 31 externally mounted at the neck portion 23 where the electron gun is housed at the position leaning toward the cathode side from the position where the deflecting yoke 30 is externally mounted and a velocity modulating coil 32 externally mounted at the neck portion 23 at the position leaning toward the cathode side from the position where the correcting magnetic device 31 is externally mounted.
The fourth electrode 4, i.e., the focusing electrode, is a relatively deep (long in the tube axial direction) cylindrical electrode as a whole and its inside is an almost equipotential space. Positive (scan direction) or negative (reverse direction from the scan direction) deflection in the horizontal scan direction acts momentarily on the electron beam passing through the fourth electrode 4 by a magnetic field caused by current flowing through the velocity modulating coil 32.
The direction of the positive deflection is the same with the horizontal deflecting direction caused by the deflecting yoke 30, so that the horizontal scan velocity of the electron beam on the screen becomes fast. The direction of the negative deflection is opposite from the horizontal deflecting direction caused by the deflecting yoke 30, so that the velocity of the electron beam on the screen becomes almost zero, thus enhancing the contrast and improving the image quality as described above.
While the velocity modulating coil 32 is mounted at any place on the way where the electron beam passes in principle, it must be mounted at the place distant from the deflecting yoke 30 by a predetermined distance so that no interference occurs with it.
Accordingly, the velocity modulating coil 32 cannot but be mounted at the place toward the cathode K rather than the fourth electrode 4, i.e., the focusing electrode. Ideally, it is disposed at the outer periphery of the fourth electrode 4 composing the focusing electrode as shown in FIG. 13.
However, because the relatively large convergence regulating and purity regulating correcting magnetic device 31 is attached to the outside of the neck portion where the fourth electrode 4 is located from the relationship of disposition of the parts at the neck portion, the velocity modulating coil 32 is attached to the position leaned toward the third electrode 3 rather than the fourth electrode 4.
Because frequency of current flowing the velocity modulating coil 32 is high and the fourth electrode 4 is made of non-magnetic metallic material such as stainless steel similarly to the other electrodes, eddy current is generated within the electrode when magnetic field acts on it from the velocity modulating coil 32.
The eddy current suppresses the magnetic flux acting on the space of the fourth electrode 4 and diminishes the velocity modulating effect.
FIG. 14 is a sectional view for explaining one structural example of a conventional electron gun. The same reference numerals therein denote the same or corresponding parts in FIG. 13. The fourth electrode (focusing electrode) 4 is divided into a first cylindrical focusing electrode 4B (fourth bottom electrode) and a second cylindrical focusing electrode 4T (fourth top electrode) in the tube axial direction.
The first cylindrical focusing electrode 4B(fourth bottom electrode) is electrically connected with the second cylindrical focusing electrode 4T(fourth top electrode) by a connecting line 7 disposed at the outside of the respective electrodes so as to have equal potential. It is noted that the third electrode 3 and the fifth electrode 5 have the equal potential, focusing voltage Vf is applied to the first cylindrical focusing electrode 4B and they are electrically connected by connectors 8, respectively.
The reference numerals 1t, 2t, 3t, 4txe2x88x921, 4txe2x88x922 and 5t are electrode supports (bead supports) for embedding and fixing the first electrode 1, the second electrode 2, the third electrode 3, the first cylindrical focusing electrode 4B, the second cylindrical focusing electrode 4T and the fifth electrode 5 to an insulating support (bead glass) 6, respectively.
The electron gun shown in the figure is a so-called large aperture single electron gun used for a projection type cathode ray tube in particular and has a large diameter portion 4F at the edge region of the second cylindrical focusing electrode 4T of the fourth electrode 4. The large diameter portion 4F is inserted to the inside of the fifth electrode 5, i.e., the anode electrode. It is noted that the cathode is not shown in the figure.
Since focusing electrode have a gap between the first cylindrical focusing electrode 4B and the second cylindrical focusing electrode 4T as shown in FIG. 14, the magnetic field from the velocity modulating coil acts directly on the electron beam.
Such arrangement allows the efficient velocity modulation to be realized by executing the velocity modulation by causing the magnetic field to enter the space of the fourth electrode 4.
Publications disclosing the prior art related to such cathode ray tube include Japanese Patent Laid-Open No. 334824/1998, 74465/1998, and 188067/2000 and Patent Publication No. 21216/1987 for example.
However, the related art cathode ray tube in which the focusing electrode is divided in the tube axial direction has had a problem that the entrance of the velocity modulating magnetic field is not enough because there is a limit in expanding the gap. That is, the influence of electric field from the bead glass and the connector becomes large if the gap between the divided electrodes is too large.
The related art cathode ray tube in which the part of the focusing electrode is coiled also has had a problem that the focusing electrode deforms, thus distorting the shape of spot of the electron beam.
Still more, no consideration is taken about the length of the divided focusing electrode in the tube axial direction in the related art cathode ray tube.
Accordingly, the invention provide a cathode ray tube comprising an electron gun which is capable of displaying high quality images by arranging the focusing electrode for modulating velocity.
The inventive cathode ray tube comprises a vacuum envelope comprising a panel portion forming a phosphor screen, a neck portion storing an electron gun and a funnel portion connecting the panel portion and the neck portion; a deflecting yoke externally mounted at the transition area of the funnel portion and the neck portion.
In the cathode ray tube, the electron gun has a plurality of electrodes including a cathode, a control electrode, an accelerating electrode, a focusing electrode and an anode electrode at predetermined intervals in the tube axial direction; the focusing electrode is composed of a first cylindrical focusing electrode which is disposed on the cathode side, a second cylindrical focusing electrode which is disposed on the phosphor screen side and at least one plate electrode or ringed electrode which is disposed between the first cylindrical focusing electrode and the second cylindrical focusing electrode; and the first and second cylindrical focusing electrodes and the plate electrode or ringed electrode are connected at equal potential by connecting line.
This arrangement also allows eddy current generated in the electrode by the magnetic field generated by the velocity modulating coil to be reduced and the magnetic field from the velocity modulating coil to readily enter to the space of the focusing electrode. Thereby, it becomes possible to obtain the full velocity modulating effect, to improve the contrast of images and to display high quality images.